The use of substitute bone tissue dates back around 1800. Since that time research efforts have been undertaken toward the use of materials which are close to bone in composition to facilitate integration of bone grafts. Developments have taken place in the use of grafts to use materials such as corals, hydroxyapatites, ceramics or synthetic materials such as biodegradable polymer materials. Surgical implants should be designed to be biocompatible in order to successfully perform their intended function. Biocompatibility may be defined as the characteristic of an implant acting in such a way as to allow its therapeutic function to be manifested without secondary adverse affects such as toxicity, foreign body reaction or cellular disruption.
Human allograft tissue is widely used in orthopaedic, neuro-, maxillofacial, podiatric and dental surgery. The tissue is valuable because it is biocompatible, strong, biointegrates in time with the recipient patient's tissue and can be shaped either by the surgeon to fit the specific surgical defect or shaped commercially in a manufacturing environment. Contrasted to most synthetic absorbable or nonabsorbable polymers or metals, allograft tissue integrates with the surrounding tissues.
Allograft bone is a logical substitute for autologous bone. It is readily available and precludes the surgical complications and patient morbidity associated with obtaining autologous bone as noted above. Allograft bone is essentially a collagen fiber reinforced hydroxyapatite matrix containing active bone morphogenic proteins (BMP) and can be provided in a sterile form. The demineralized form of allograft bone is naturally both osteoinductive and osteoconductive. The demineralized allograft bone tissue is fully incorporated in the patient's tissue by a well established biological mechanism. It has been used for many years in bone surgery to fill the osseous defects previously discussed.
Alograft bone occurs in two basic forms; cancellous and cortical. The cancellous bone includes void areas with the collagen fiber component contributing in part to torsional and tensile strength. The less dense cancellous bone provides an excellent matrix for rapid bone regeneration and repair.
Many devices of varying shapes and forms are fabricated from allograft cortical tissue by machining. Surgical implants such as pins, rods, screws, anchors, plates, intervertebral spacers and the like have been made and used successfully in human surgery. These pre-engineered shapes are used by the surgeon in surgery to restore defects in bone to the bone's original anatomical shape.
Injury or disease processes to the head, neck, or shoulders can cause abnormal forces to be applied on the cervical vertebra. Arthritis, motion induced “whiplash”, or other trauma create this malfunction. This situation is often treated surgically by a procedure intended to fuse the two adjacent cervical or spinal vertebrae to each other. Such fusion relieves the pressure the partially displaced vertebrae place on the adjacent spinal nerves.
Many surgical devices have been developed and used successfully to immobilize and fuse the misaligned vertebrae. Metal plates screwed into the adjacent vertebrae work well, but after some time post-operatively, the stress rise occurring at the screw position causes erosion of the bone and resultant slipping. This has been improved by placing load-bearing spacers between the two (or more) misaligned vertebrae. The spacer is both load-bearing and of a material which will induce, or at least support, fusion between the vertebrae.
Removal of damaged or diseased discs, restoration of disc space height and fusion of adjacent vertebrae to treat chronic back pain and other ailments are known medical techniques. Implants such as intervertebral spacers are often implanted in the disc space engaging the vertebrae to maintain or reestablish disc space height after removal of all or a portion of the disc. The spacers are formed of a variety of both resorbable and non-resorbable materials, including, for example, titanium, surgical steel, polymers, composites and bone. It is currently considered desirable to promote fusion between the vertebral bodies that are adjacent to the damaged or diseased discs. Typically, an osteogenic material is combined with a spacer and inserted in the disc space to facilitate and promote bone growth. While the selection of the implant configuration and composition can depend upon a variety of considerations, it is often desirable to select a resorbable material that does not shield the bone ingrowth. Bone and bone-derived components can provide suitable material to prepare the implants. However, bone material and in particular cortical bone acceptable for use in implants is a scarce resource, being derived from limited number human tissue donor resources.
Suitable bone or bone-derived material for use in implants, in general, is almost exclusively obtained from allograft and xenograft sources, both of which come from a limited supply. Since intervertebral spacers must withstand the compressive loads exerted by the spine, these implants are often cortical bone which has the mechanical strength suitable for use in any region of the spine. Cortical spacers are often shaped from cortical long bones, which are primarily found in the lower limbs and include, for example, femur, fibula, and the tibia bones. However, these long bones make up only a fraction of the available bone source. Cancellous bone, because of its superior osteoinductive properties, would be desirable to sue in the spinal implant. However, the lower mechanical strength of cancellous bone prohibits its use in many surgical applications. Thus, sources of bone suitable for structural intervertebral spacers are extremely limited. The scarcity of desired donor bone makes it difficult to provide implants having the desired size and configuration for implantation between vertebrae, which can require relatively large implants. It is further anticipated that as the population ages there will be an increased need for correction for spinal deformities and a concomitant increase in the demand for bone-derived components. Therefore, these structural bone portions must be conserved and used efficiently to provide implants. The scarcity of suitable bone material has also hindered efforts to design and manufacture varying configurations of suitable implants for arthodesis of the spine. Further, various implant configurations have not been physiologically possible to obtain given the structural and geometrical constraints of available donor bone.
One known treatment for fusing two vertebrae is the insertion of a suitably shaped dowel into a prepared cylindrical cavity which reaches the two vertebrae to be fused. The dowel used is preshaped bone or allograft bone.
A number of allograft bone spacers have been used in surgery as spacers. They are commonly called the ACF spacer constructed as a cortical bone cross section, shaped like a washer with teeth to discourage graft explusion and an axial center hole; a VG3 cervical spacer constructed with two ramp shaped cortical plates held together with cortical pins, the top and bottom surfaces being ridged to discourage graft expulsion; an ICW spacer constructed with an elongated C spaced cortical portion with a cancellous inside to allow rapid ingrowth (slice of iliac crest) and a SBS spacer constructed with a single piece cortical member with serrated top and bottom surfaces and an axial center hole.
The ICW (iliac crest wedge) has been used for a long time for cervical spine fusion and has a total load bearing force around 4500 Newtons. Testing has noted that cervical vertebrae fail in compression at about 2000 Newtons. The ICW spacer suffers from high unit variability because of its natural, anatomic variations.
U.S. Pat. No. 5,972,368 issued on Oct. 26, 1999 discloses the use of cortical constructs (e.g. a cortical dowel for spinal fusion) which are cleaned to remove all of the cellular material, fat, free collagen and non-collagenous protein leaving structural or bound collagen which is associated with bone mineral to form the trabecular struts of bone. The shaped bone is processed to remove associated non-collagenous bone proteins while maintaining native bound collagen materials and naturally associated bone minerals. The surface of a machined cortical bone is characterized by a wide variety of openings resulting from exposure by the machining process of the Haversian canals present throughout cortical bone. These canals serve to transport fluids throughout the bone to facilitate the biochemical processes that occur at variable angles and depths within the bone.
An attempt to solve the increasing bone supply problems using a combined cortical and cancellous bone block is shown in U.S. Pat. No. 4,950,296 issued Aug. 21, 1990 which uses a cubically configured cortical shell defining a through going internal cavity and a cancellous plug fitted into the cavity so that the end surfaces of the cancellous plug are exposed. Another reference, WIPO Patent Publication Number WO 02/24122 A2, published Mar. 28, 2002 owned by SDGI Holdings Inc. show various intervertebral spacers formed of cortical and cancellous bone composites such as sandwiches, with intersecting ribs and rods.
U.S. Pat. No. 6,294,187 issued Sep. 25, 2001 is directed toward an shaped osteimplant of compressed bone particles. The shaped implant is disc shaped and has a number of holes drilled therein for macroporosity and the holes can be filled with an osteogenic putty material.
Conversely, WIPO Patent Publication Number WO 02/07654 A2, published Jan. 31, 2002 discloses intervertebral spacers formed of dense cancellous human or animal bone. In one embodiment, a cortical rod or cortical rods are placed in bores cut through a cancellous bone block to provide load bearing strength with the ends of the rods being exposed on both sides of the cancellous bone block. Another embodiment shows a C shaped cortical block with a cancellous plug inserted into the recess of the C to form a rectangular spacer. A pin is inserted through a bore cut through the legs of the C block and through the cancellous plug to keep the cancellous plug positioned with the recess of the cortical component. U.S. Pat. No. 6,379,385 issued Apr. 30, 2002 also discloses the use of a spongy block having a plurality of cortical rods mounted in through going bores cut through the bone block. In another embodiment, a X-shaped cortical support member is mounted therein to provide structured strength to the composite implant.
It is also known to mate various bone components together to form a single implant. In this regard, see, Albee, Bone Graft Surgery in Disease, Injury and Deformity, (1940), pp. 30, which uses a tongue in groove and dove tail to hold separate pieces of bone together for implant use, and U.S. Publication No. US2002/0029084 A1, published Mar. 7, 2002, which shows a three component implant with a center core surrounded by two outer semicircular portions. The outer portions have alternative dove tail joints on adjacent bone portions to secure the outer portions together forming a dowel shaped bone implant.
Consequently, there is a need for an implant which should have with a load bearing compressive strength of 1000 to 5000 Newtons with a compressive load to be a minimum of 3000 Newtons as a safety factor. There is also a need to have a portion of cancellous bone immediately adjacent to the load bearing cortical zone to permit rapid ingrowth of a patient's own new bone with the cancellous bone forming the major part of the implant.